1. Field of the Invention
The present invention relates to organic electroluminescent (EL) elements, and more specifically, to organic EL elements that have a structure for extracting guided wave light in a front direction.
2. Description of the Related Art
In order to reduce the power consumption of organic EL elements, it is necessary to increase the emission intensity of the organic EL elements. A known method increases the emission intensity of an organic EL element by extracting light trapped in the organic EL element, due to total internal reflection, among light emitted from a light-emitting layer of the organic EL element.
FIG. 10 is a schematic cross-sectional view of an organic EL element 101 of the related art. Light entrapment due to total internal reflection will be described by referring to FIG. 10. Typically, a substrate 102 is made of glass (having a refractive index of about 1.5), a transparent electrode 106 is made of indium tin oxide (ITO) or indium zinc oxide (IZO™) (having a refractive index of about 1.9 to 2.1), and a light-emitting layer 105 inside an organic EL layer 104 is made of Alq3 or the like (having a refractive index of about 1.7 to 1.9). These materials are stacked substantially in parallel. When light passes from a medium having a high refractive index to a medium having a low refractive index, total internal reflection occurs, according to Snell's law, at an incident angle exceeding the critical angle. In the organic EL element 101, roughly two total internal reflections occur.
A first total internal reflection occurs at the interface between the transparent electrode 106 and the substrate 102, because the substrate 102 has a lower refractive index than the light-emitting layer 105. Because the totally internally reflected light is transformed into propagation light 111 propagating along the surface of the substrate 102 while being repeatedly reflected by the interface between the substrate 102 and the transparent electrode 106 and the interface between the reflective electrode 103 and the organic EL layer 104, the light is trapped in the organic EL element 101. Since this propagation light 111 is not extracted to an outer area 110 (air) of the organic EL element 101, it decreases the emission intensity of the organic EL element 101. Thus, in order to increase the emission intensity, a medium having a lower refractive index than the light-emitting layer 105 should not be provided between the light-emitting layer 105 and the outer area 110.
A second total internal reflection occurs at the interface between the substrate 102 and the outer area 110, because the outer area 110 (air) has a lower refractive index than the light-emitting layer 105. Because the totally internally reflected light is transformed into propagation light 111′ while being repeatedly reflected by the interface between the substrate 102 and the outer area 110 and the interface between the reflective electrode 103 and the organic EL layer 104, the light is trapped in the organic EL element 101. In order for the light emitted from the organic EL element 101 to be viewed by human eyes, the light needs to be extracted to the outer area 110. Therefore, substitution by a material having a high refractive index, as in the case of the first total internal reflection, cannot be used. Thus, in order to increase the emission intensity, it is necessary to prevent the interface between the organic EL element 101 and the outer area 110 from satisfying the total-internal-reflection condition by changing the angle of the interface between the organic EL element 101 and the outer area 110 or by causing the light to be scattered at the interface.
Akiyoshi Mikami SID '09 DIGEST P. 907 60.4 L proposes a configuration for reducing the two total internal reflections mentioned above, in which the substrate is made of a material having a higher refractive index (2.0) than the light-emitting layer so that there is no material having a lower refractive index than the light-emitting layer between the light-emitting layer and the outer area of the organic EL element, and in which spherical lenses are formed on the substrate, at the interface with respect to the outer area. According to the report, this configuration improves the efficiency of extracting light to the outside.
Japanese Patent Laid-Open No. 2005-055481 proposes a configuration for reducing the above-mentioned second total internal reflection, in which square-pyramid-shaped light extraction structures, which are three-dimensional bodies having inclined portions, are provided on the organic EL element, at the interface with respect to the outer area. The light extraction structures having the inclined portions provided on a light-exiting side of the organic EL element change the angle of the interface between the organic EL element and the outer area with respect to the guided wave light. Japanese Patent Laid-Open No. 2005-055481 improves the efficiency of extracting light to the outside by setting the angle of the inclined portions such that total internal reflection is less likely to occur.